1. Field of the Invention
The present invention relates to a single crystalline aluminum nitride (AlN) film, a method of forming the same, which can ensure highly efficient formation of an aluminum nitride film, a base substrate for a Group III element nitride film, a light emitting device and a surface acoustic wave device.
2. Description of the Prior Art
Special attention has recently been paid to group III element nitride semiconductors typified by a gallium nitride (GaN)-based semiconductor as materials for light emitting devices such as a blue light emitting diode (LED) and a blue laser. As seen in the laminated structure of a blue LED, a group III element nitride having a high melting point must be epitaxially grown on a substrate such as a sapphire substrate. Due to its great lattice mismatching with the material of the substrate, it is still extremely difficult to obtain a group III element nitride thin film having few defects. Since the initial crystal growth on the substrate is the major factor in determining the luminous efficiency of a group III element nitride-based semiconductor device, the development of substrate materials which have excellent lattice matching with the above nitride is the most important matter that will bring a major breakthrough in this field.
For the purpose of suppressing lattice mismatching between a sapphire substrate and a group III element nitride, there has been proposed to insert a so-called buffer layer of AlN or the like between the sapphire substrate and the group III element nitride film.
However, since the buffer layer of AiN or the like also has great lattice mismatching with the sapphire substrate, it is difficult to obtain a defect-free homogeneous thin film. In the prior art, a buffer layer is formed on a sapphire substrate by molecular beam epitaxy (MBE), halide vapor-phase epitaxy (HVPE) using aluminum chloride and ammonia, or metal organic vapor-phase epitaxy (MOVPE) using trimethyl aluminum and ammonia. In all of the above methods, large distortion remains at the junction interface between the sapphire substrate and the thin MN film due to great lattice mismatching therebetween. Therefore, the AiN film formed on sapphire becomes a frost column-like assembly structure with a high dislocation density. Accordingly, the conventional AlN film does not sufficiently serve as a buffer layer for growing a GaN film and the GaN film has threading dislocations that thread through the substrate to the film surface at a density of 108 to 107/cm2. This is the main factor that lowers the luminous efficiency of a light emitting device. As a result, the conventional blue LED has a low luminous efficiency of 22% and the conventional ultraviolet LED has a low luminous efficiency of only 7.5%.
As means of suppressing lattice mismatching at the time of depositing GaN on an MgAl2O4 spinel substrate, there has been proposed a technique which uses an aluminum oxynitride layer as a buffer layer (U.S. Pat. No. 5,741,724). However, as this prior art technique uses a bulky apparatus for MOVPE, MBE or the like to form an aluminum oxynitride layer and forms a GaN film on the aluminum oxynitride layer, it cannot be said that lattice mismatching has been improved sufficiently.
There has been further proposed a technique for a single crystalline laminated substrate comprising a sapphire substrate, an aluminum oxynitride film formed on the substrate and further an aluminum nitride single crystalline thin film formed on the above film (JP-A 2-141495 and JP-A 2-153897) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). This technique is to grow the aluminum oxynitride film and the aluminum nitride single crystalline thin film on the sapphire substrate by CVD. The formed aluminum oxynitride film has a non-equilibrium phase called xe2x80x9cdistortion superlatticexe2x80x9d in which the concentration of oxygen on the sapphire substrate side produced at a substrate temperature of 1,150xc2x0 C. is 25 mol % and that on the aluminum nitride side is 0 mol %. This substance differs from cubic xcex3-spinel type aluminum oxynitride existent at 1,630xc2x0 C. or higher obtained in the present invention. The aluminum nitride film formed through this aluminum oxynitride film has been evaluated with the half width of an X-ray rocking curve alone and it is not confirmed yet whether the sample is totally single crystalline or not.
It is an object of the present invention which has been made in view of the above problems to provide a single crystalline aluminum nitride film which has a low dislocation density, little lattice mismatching and excellent crystallinity and can ensure the formation of a Group III element nitride film excellent in luminous efficiency, a method of forming the same, a base substrate for a Group III element nitride film, a light emitting device and a surface acoustic wave device.
The inventors of the present invention have conducted studies on thermodynamics of an aluminum nitride formation reaction using alumina, carbon and nitrogen as reaction raw materials and have found that a single crystalline aluminum nitride film having excellent crystallinity can be formed by converting an alumina component into aluminum oxynitride and aluminum nitride from the surface of a sapphire substrate toward its interior, making use of the above equilibrium reaction and not by forming a thin film of interest on a sapphire substrate by deposition as in the prior art. The present invention has been thus accomplished based on this finding.
According to the present invention, firstly, the above object of the present invention is attained by a single crystalline aluminum nitride laminated substrate comprising a single crystalline xcex1-Al2O3 substrate, an aluminum oxynitride layer formed on the substrate and a single crystalline aluminum nitride film as the outermost layer, wherein the density of dislocations in the single crystalline aluminum nitride is 108/cm2 or less.
According to the present invention, secondly, the above object of the present invention is attained by a process for producing a single crystalline aluminum nitride laminated substrate, comprising nitriding a single crystalline xcex1-Al2O3 substrate to form an aluminum oxynitride layer and a single crystalline aluminum nitride film on the aluminum oxynitride layer so as to produce the above single crystalline aluminum nitride laminated substrate of the present invention.
According to the present invention, thirdly, the above object of the present invention is attained by a method of forming a single crystalline aluminum nitride film, comprising nitriding a single crystalline xcex1-Al2O3 substrate to form an aluminum oxynitride layer and an aluminum nitride film on the aluminum oxynitride layer.
According to the present invention, in the fourth place, the above object of the present invention is attained by a single crystalline aluminum nitride film which can be obtained by the above method of the present invention.
According to the present invention, in the fifth place, the above object of the present invention is attained by a base substrate for a GaN film, which comprises the above single crystalline aluminum nitride laminated substrate of the present invention.
According to the present invention, in the sixth place, the above object of the present invention is attained by a light emitting device which comprises the above single crystalline aluminum nitride laminated substrate of the present invention including a single crystalline aluminum nitride film as a light emitting film.
According to the present invention, in the seventh place, the above object of the present invention is attained by a light emitting device having a semiconductor film made from group III element nitride single crystals on the above single crystalline aluminum nitride laminated substrate of the present invention as a light emitting film.
According to the present invention, in the eighth place, the above object of the present invention is attained by a surface acoustic wave device which comprises the above single crystalline aluminum nitride laminated substrate of the present invention.
In the present invention, an aluminum oxynitride (alon) layer is formed on the surface of a single crystalline xcex1-Al2O3 substrate, for example, a sapphire substrate and a single crystalline aluminum nitride (AlN) film is formed on this alon layer by nitriding the substrate. Therefore, the obtained single crystalline AlN film has an extremely low dislocation density and excellent crystallinity. Accordingly, when this single crystalline AlN film is used as a light emitting device, an ultraviolet light emitting device having extremely high luminous efficiency can be obtained and can be used in a high-density optical memory and also in a wide variety of fields as an ultraviolet light receiving device. Further, when a group III element nitride semiconductor typified by a GaN-based semiconductor is epitaxially grown on this single crystalline AlN film having excellent crystallinity, a group III element nitride semiconductor single crystalline film having extremely high crystallinity and few defects can be formed. Therefore, a blue light emitting device or ultraviolet light emitting device comprising this group IIII element nitride semiconductor single crystalline film has greatly improved luminous efficiency. Further, since single crystalline AlN can be used in a surface acoustic wave device and a single crystalline AlN film having excellent crystallinity is obtained by the present invention, the drive efficiency of a surface acoustic wave device comprising this film can be enhanced.